Shredded scrap metal is produced typically by feeding obsolete or discarded articles of metal (i.e., scrap metal) into an apparatus called a shredder in which the scrap metal is flailed, by rotating, free-swinging hammers, into relatively small, fist-sized pieces that, after further processing, provide a densified scrap charge in a melting furnace. A typical feed into a shredder comprises whole junk autos, discarded appliances, scrap metal recovered from demolition projects, light gauge other obsolete steel scrap and some heavier obsolete steel scrap.
The output product of the process described above is a shredded mixture comprising principally shredded ferrous scrap metal plus (a) some shredded non-ferrous scrap metal (e.g., aluminum, zinc-base alloys, copper and copper-base alloys), (b) some stainless steel and (c) up to about 25% of non-metallic material (plastic, rubber, fabric and the like). The shredded ferrous scrap metal is magnetically separated from the other materials. Other procedures, downstream of the shredder, are employed to separate the non-metallic material from the other metallics and to separate the aluminum from the other non-ferrous metals.
Shredders and their operation are described in detail in the following publication: Nijkerk, A. A. and Dalmijn, W. L., Handbook of Recycling Techniques, Nijkerk Consultancy, The Hague, Netherlands, 1998, pp. 88-109 (hereinafter referred to as Nijkerk).
Shredded ferrous scrap unavoidably contains pieces of tramp, non-ferrous scrap that are physically attached to or entangled with pieces of ferrous scrap by fasteners or the like, or otherwise attached, and as a result, have been carried over with the ferrous scrap pieces to which they were attached, during magnetic separation. Included among the attached and entangled non-ferrous scrap pieces are pieces of copper or copper alloys, mostly in the form of copper wire entangled with ferrous pieces or copper wire wound around the iron armatures of electric motors, particularly when the armatures are from small electric motors such as those used to operate automobile windshield wipers and window openers (see Nijkerk supra, p. 107).
Copper is undesirable as a constituent in a steel melting furnace charge for making a product known as flat rolled steel which is flawed by the presence of copper in excess of a specified maximum amount above which the steel is rendered unfit for its intended uses (e.g., deep drawing). Copper cannot be removed from a bath of melted steel by refining. If an undesirable amount of copper is present as an impurity in a steel melt, the copper content has to be diluted by the addition of relatively expensive, low copper-content scrap, such as bales of compressed, flat-rolled steel scrap salvaged as clippings or the like from industrial stamping plants and which typically contain less than 0.10% copper; these are known as factory bundles or number one bundles.
There are some steel products that are unaffected by the presence of copper in amounts that adversely affect flat rolled steel. These products include steel structural shapes (i.e., I-beams and the like), steel plate and, most particularly, steel reinforcing bars. Steel mills making these products can tolerate, in their melting furnace charge, steel scrap containing up to 0.20-0.25% copper, for example, and even higher copper percentages when the intended end product is steel reinforcing bars. Some or much of the copper and other non-ferrous materials carried over from the magnetic separation step are removed at a picking conveyor located downstream of the magnetic separator. At the picking conveyor, pieces of ferrous scrap with physically attached or entangled copper (or other non-ferrous materials) are visually located and manually removed from a moving stream of shredded ferrous scrap. This removes at least some of that copper, but other pieces of attached or entangled copper may escape removal. Copper present in shredded ferrous scrap as attached or entangled copper or copper alloy will sometimes be referred to herein as free copper or as unincorporated copper.
In addition to copper of the unincorporated kind, copper can be present metallurgically in ferrous scrap as an internal impurity, or even as an alloying addition. Copper is widely employed as an alloying addition to improve the resistance to atmospheric corrosion of both plain carbon steel and alloy steels. When so employed, the copper content of the steel is at least 0.20% and up to 1.0% or more. Copper present in shredded ferrous scrap as an internal impurity in steel or as an alloying addition in steel will sometimes be referred to herein as incorporated copper or metallurgically incorporated copper.
Given the considerations noted above, it is important for both the supplier of shredded ferrous scrap and the steel mill consumer of shredded ferrous scrap to know the copper content of that scrap, especially when the scrap is intended as part of the charge for making a flat rolled steel product.
To this end, equipment has been developed that enables one to analyze a moving stream of shredded ferrous scrap carried on a conveyor. The analyzing equipment surrounds the conveyor and provides a real-time analysis of the chemical composition of the stream as it is conveyed through the analyzing equipment. An information processor associated with the analyzing equipment calculates the average content of the copper (and other impurities and alloy constituents) over a set number of tons (e.g., an entire stockpile or a barge load).
A person monitoring the real-time analysis can detect when the shredder is producing shredded steel scrap with an undesirably high copper content and can alert the shredder operator who can make processing adjustments that increase the density of the shredded scrap by grinding it to a finer size; this reduces the amount of copper attached to pieces of shredded ferrous scrap, thereby lowering the average copper content of the ensuing batch of a set number of tons of shredded ferrous scrap.
However, practical considerations preclude lowering the copper content of a batch of several tons of shredded ferrous scrap to that of factory bundles (0.10% Cu or less). More typically, an average copper content for a relatively large batch (e.g., a barge-load of 1,300 tons) processed in the manner described above, would be, e.g., about 0.14%.
Nevertheless, because scrap suppliers employing the analyzing and information processing equipment described above, can guarantee the copper content of a load of shredded steel scrap delivered to a maker of flat-rolled steel, that steel maker will pay a premium for the scrap. The premium is substantially less than that paid for factory bundles, but it is sufficient to justify the extra capital and operating expenses the scrap supplier undergoes in order to provide the guarantee.
Shredders were first used to process junk autos and the like in the late 1950s (see Nijkerk, supra, pp. 88-92), and shredders have been in widespread use for the processing of ferrous scrap for several decades; but the problem of excess copper content in shredded ferrous scrap has still not been solved to the entire satisfaction of consumers of that scrap. In this regard, see the discussion entitled “Achieving Purer Ferrous Shred” in the trade journal Scrap, January-February 2007, pp. 107-108, which also describes, at p. 108, some attempts to deal with this problem.
The analyzing equipment described above is known as a prompt gamma neutron activation analyzer or PGNAA. This equipment, and its operation in connection with the analyzing and processing of copper-containing, shredded ferrous scrap, are described in detail, in the trade journal Scrap, November-December, 2006, pp. 71-78, and the description therein is incorporated herein by reference.
In Pflaum U.S. Pat. No. 5,948,137, a PGNAA is employed to analyze batches of steel scrap that are used to make up a steel melting furnace charge that conforms to a predetermined reference composition.
Additional detailed descriptions of a PGNAA and its operation are contained in Atwell et al. U.S. Pat. Nos. 5,732,115 and 4,582,992.
Pederson et al. U.S. published patent application US 2006/0115037 describes a PGNAA included in an apparatus that analyzes and separates, on a piece-by-piece basis, chemically treated timber from untreated timber, and PVC plastic from other plastic in a waste stream.
Clayton et al. U.S. Pat. No. 4,830,193 and European published application No. EP 059,033 each describe an analyzer, in some respects similar to a PGNAA, that is employed as part of an apparatus that analyzes and sorts, on a piece-by-piece basis, particles or lumps of gold ore.
There are reports of shredded scrap aluminum undergoing analysis and sorting, on a piece-by-piece basis, using laser induced breakdown spectroscopy (LIBS) to perform the analysis. Nijkerk, supra, at pp. 201-202, describes LIBS in detail in connection with the analysis and sorting, on a piece-by-piece basis, of shredded scrap aluminum and other non-ferrous scrap particles.
A sorter using x-ray transmission (XRT) separates shredded, low-density aluminum fragments from shredded fragments of higher density, non-ferrous metals in a mixture of shredded non-ferrous metals in which the shredded fragments have been aligned and spaced apart. The XRT sorter differentiates among the fragments on the basis of density, and the fragments are analyzed and separated on a piece-by-piece basis.
A mixture of shredded fragments of various non-ferrous metals can be sorted using x-ray fluorescence (XRF) which differentiates among the fragments on the basis of chemical composition. When one employs an XRF sorter, the fragments are aligned and spaced apart, and they are analyzed and separated on a piece-by-piece basis.
It has been reported that efforts are underway to develop an XRF sorter to remove, from shredded ferrous scrap, free and commingled copper including electric motor armatures comprising copper wire wound around an iron core (called “meatballs” in industry jargon).
A mixture of shredded scrap fragments containing two different metallic components (e.g., copper and stainless steel) can be separated into the mixture's two individual components using a piece of equipment known as a sensor sorter. The metallic fragments are aligned and spaced apart, and the fragments are analyzed and separated on a piece-by-piece basis.
When XRT, XRF and sensor sorters are used to sort a mixture of shredded non-ferrous fragments, individual fragments of one composition are sorted from other fragments in the mixture by air jets or mechanical fingers. Additional descriptions of XRT, XRF and sensor sorters are contained in the trade journal Scrap, November-December, 2007, pp. 113-126.
Non-ferrous sorting systems that analyze or detect individual, spaced apart fragments of shredded scrap and sort them with air jets or mechanical fingers, on a piece-by-piece basis, need to screen for size the particles of shredded scrap and maintain a particle size within maximum and minimum limits, or the sorting accuracy suffers (Id., p. 120).
There is a published paper describing the processing of a moving stream of coal wherein coal from the stream is analyzed, in a side-stream sampling loop, by a bulk material analyzer, particularly for sulfur and ash contents. (Woodward, R., et al., “Automated Coal Blending With Two Control Parameters at One of Arch Coal's West Virginia Mining Complexes”, International On-Line Coal Technical Conference, St. Louis Mo., Nov. 8-10, 2004. htpp://www.thermo.com/e_Thermo/cma/PDFs/Various/File—24617.pdf).
Nijkerk, supra, at pp. 127-128, describes a shredding system for aluminum in which pieces of shredded aluminum containing attached pieces of iron are separated (presumably by magnetic separation) from pieces of shredded aluminum that are free of attached iron, and the pieces with attached iron are recycled to the shredder for further processing to remove the attached iron (Id., FIG. V-11-20, and p. 128).
Referring again to the moving stream of shredded ferrous scrap that undergoes analysis with a PGNAA, in one embodiment the moving stream is typically about six feet wide, eight to twelve inches deep and is typically trough-shaped in lateral cross-section (Scrap November-December, 2006, supra, pp. 72, 76). The stream flows through the analyzer typically at a rate of, e.g., 250-300 tons per hour (Ibid.).
In accordance with prior art procedure, and as previously noted, there is an information processor associated with the bulk material analyzer (PGNAA), and this information processor calculates the average composition for a set number of tons, e.g., the tonnage flowing through the analyzer in a given period of time (e.g., 250-300 tons in one hour). Such a tonnage will sometimes be referred to hereinafter as a “batch”. As previously noted, there is a calculated average composition for the entire batch, (e.g., 0.14% copper); however, the average composition for an increment of the stream having a relatively small length (e.g., one foot or three feet or five feet, etc.) may or may not be the same as the average composition for the batch. Some of the increments may contain certain ingredients having an average composition corresponding roughly to the aforementioned average composition of the batch (e.g., a copper content of 0.14±0.02%); other increments may have an average composition substantially less than that of the batch (e.g., a copper content of 0.10% or less); and there may be still other increments that have an average composition substantially greater than that of the batch (e.g., a copper content of 0.20-0.25%, or more). Even in those streams from which virtually all pieces of attached or entangled copper or copper alloy have been visually located and manually removed at the picking conveyor, or have been otherwise detected and removed (e.g., by XRF sorting), individual increments in the stream can have average copper compositions substantially greater than that of the batch due to the presence in such increments of steel fragments containing metallurgically incorporated copper (i.e., copper present in the steel as an impurity or as an alloying agent). The presence of metallurgically incorporated copper also prevents one from reducing the average copper content of the batch generally to less than 0.12% to 0.14%, no matter the efficiency of the shredding and free copper-removing procedures employed upstream of the analyzer.
As noted above, equipment has been developed that provides a prompt, real-time analysis of a stream of shredded ferrous scrap metal, but that equipment (PGNAA) does not sort the scrap; the ability to do so would be desirable.